1. Field of the Invention
The present invention relates to a method for forming a photoluminescence layer on a semiconductor layer, and more specifically to a method for forming a photoluminescence semiconductor layer even in a small area or even in large area, with a high reproducibility, a high uniformity, and an excellent mass producibility.
2. Description of Related Art
In the prior art, as regards a photoluminescence semiconductor, there have been many reports using a direct transition semiconductor such as GaAs. In order to form a photoluminescence layer by an indirect transition semiconductor such as Si, it has been foraged by an anodizing using an electrolytic solution mainly composed of hydrofluoric acid and using a silicon substrate as an anode.
For example, an electrode is formed on a rear surface of a substrate in order to increase uniformity of the anodizing. This electrode can be formed by depositing aluminum on the rear surface of the substrate and then by performing a beat treatment at 500.degree. C. for one hour. Thus, the substrate is used as an anode, and a cathode formed of platinum is used. The electrolytic solution is composed of a 49%-aqueous hydrofluoric acid or a mixed liquid of 5 parts of 49%-aqueous hydrofluoric acid and 3 parts of ethanol. A current is applied with a current density of 10 mA/cm.sup.2 to 50 mA/cm.sup.2 for a few minutes to 30 minutes. Thus, a photoluminescence layer is formed.
In accordance with the an present invention, in order to obtain a uniform photoluminescence region, it is necessary to form an electrode having a uniform contact resistance on the silicon substrate.
However, it is difficult to form the electrode of uniform contact resistance over a large area, and as a result, it is difficult to uniformly anodize the silicon substrate surface. In addition, since the reaction is performed in the solution, a non-uniformity occurs in solution concentration in the process of reaction, and hydrogen bubbles are generated in the proximity of the electrode within the solution also in in the process of reaction. Because of these factors, it is difficult to form a uniform anodized surface.
Accordingly, it is difficult to form a semiconductor region having a uniform photoluminescence intensity. In addition, since it is necessary to form the electrode on the semiconductor substrate, the producibility is low. Furthermore, at the time of forming the electrode on the semiconductor substrate, there is a risk that the substrate is soiled and damaged. In addition, the electrode makes it impossible to form a device on the substrate or makes it necessary to remove the electrode.
From a different viewpoint, there is no attempt for wholly or selectively changing the photouminescence intensity after the photoluminescence region is formed, or for restoring the photoluminescence intensity in the region of reduced or inhibited photoluminescence intensity.